Electrical welding machine having amperage control transformer

ABSTRACT

An arc welding unit operable on single or multiple phase alternating current and including for each phase a basic control device including a three-legged magnetic core. A primary leg includes a portion of the secondary, or output, winding, the remainder of which is wound on a secondary leg of the core. Flux through the secondary leg is controlled by regulating the duty cycle of the saturation current through a control winding on the third leg of the core, and flux-induced emf in the secondary winding due to current flow through the control winding is essentially eliminated because the secondary is formed on both the primary and secondary legs.

United States Patent Me ia [451 May 23, 1972 ELECTRICAL WELDING MACHINE[56] Rele'wm Cited g CONTROL UNITED STATES PATENTS 3,059,170 10/1962Jetter ..219/131 X I [72] Inventor: Salvador Mejia, Mexico D. F., Mexico2,644,109 6/ l953 Mulder ..219/131 X Assigneez Indusflas Si SA. e CAI,colonia 3,493,718 2/1970 Kestner et al. ..2l9/l3l R lndustnal Vallejo,Mexico, D.F., Mexico Pn-mary Examiner R' R Staubly [22] Filed: Sept. 18,1970 Attorney-Thomas M. Marshall 21] Appl. NOJ 73,215 57 I ABSTRACT Anarc welding unit operable on single or multiple phase alter- [301Foreign Application Prmmy Data nating current and including for eachphase a basic control Nov. 21 1969 Mexico ..115 484 device includinB afluee'legged magnetic A Primary v 7 includes a portion of the secondary,or output, winding, the

52 US. (:1 ..219 131 WR 19 135, 336 170, remind" which is 3 secndary leg1 I 6 3360215 Flux through the secondary leg is controlled by regulatingthe 51 1 1111.01 ..112311 9/10 11611140108 duty cycle current a 58 Fieldof Search ..219/131 R, 131 I11, 132 135; leg and flux'induced emf sewndary winding due to current flow through the control winding isessentially eliminated because the secondary is formed on both theprimary and secondary legs.

4 Claim, 12 Drawing Figures Patented May 23,1972 3,665,150

12 Sheets-Sheet 1 SALVATOR MI'JJIA ATTORNEY Patented May 23, 19723,665,150

12 Sheets-Sheet 2 FIG. 2

INVENTOR SALVATOR MEJIA ATTORNEY Patented May 23, 1972 1.2 Sheets-Sheetl5 INVENTOR SALVATOR MEJIA ATTORNEY Patented May 23, 1972 12Sheets-Sheet 4 INVENTOR SALVATOR MEJIA ATTORNEY Patented May 23, 19723,665,150

12 Sheets-Sheet 5 INVENTOR SALVATOR MEJIA ATTORN E Y Patented May 23,1972 3,665,150

12 Sheets-Sheet 7 INVENTOR SALVATOR MEJIA ATTORNEY Patented May 23, 19723,665,150

12 Sheets-Sheet 8 n g l 44: 7 2. /l E 46 FIG. 8 P

INVENTOR SALVATOR MEJIA ATTORNEY Patented May 23, 1972 3,665,150

12 Sheets-Sheet O INVENT( )R SALVATOR MEJIA ATTORNEY Patented May 23,1972 3,665,150

12 Sheets-Sheet 1O lo 0 l 12' 16 15' 13' 12 16 15 13 18 17 1 8 17 r 1 W1 T FIG. IO

i INVENTOR SALVATOR MEJIA Patented May 23, 1972 3,665,150

12 Sheets-Sheet l1 INVENTOR SALVATOR MEJIA ATTORNEY ELECTRICAL WELDINGMACHINE HAVING AMPERAGE CONTROL TRANSFORMER BACKGROUND OF THE INVENTIONThe present invention relates to an improved system for controllingwelding current in electric welding machines.

There are currently many welding machines with different types ofwelding current control. One such known system for controlling thewelding amperage is mechanical and may implement a moving coil, movingcore or moving reactor. In those machines, however, all parts subject tomovement may undergo excessive wear, causing misadjustments, vibrationsand clogging of the machine due to dust and foreign atmospheric agents.Moreover, they tend to have a high cost.

Another type of system is the electromagnetic type, which operates bychanging the magnetic coupling between the primary and the secondarywindings of a transformer by means of taps, either on the primary sideor on the secondary side, orthrough manual plugs, switches or selectors.This type of control is the simplest among the electromagnetic type butsuffers the following shortcomings: a reduced number of limited weldingamperages, possible different open-circuit voltages on each tap, faultycontacts in the plugs dueto wear and clogging due to dust or atmosphericagents.

A different version of the electromagnetic control uses a saturablereactor in series with the secondary of the main transformer, andcontrol of the welding current is obtained by modifying the saturationof the reactor. Disadvantages of this type of control include excessiveweight of the machine attributable to large saturable reactors, and thehigh cost of auxiliary windings used in the saturable reactor. On theother hand, it has the advantage of not having moving parts which can besubjected to wear. f

It is consequently among the objects of the present invention to avoidthe deficiencies of the prior art devices and to provide aweldingmachine which requires no moving parts, is lightweight, simpleand economically constructed and is capable of obtaining an infiniterange of amperages selectable by remote control adjustment of, forexample, a potentiometer of reduced dimensions.

SUMMARY OF THE INVENTION In brief, the foregoing objects are attained byan electric welding machine incorporating a current control devicehaving primary means for establishing flux in a magnetic circuit, meansin magnetic circuit with the flux to provide an output current, andcontrollable means magnetically coupled to the primary means foraltering the flux in the magnetic circuit and thereby controlling theoutput current. In a preferred embodiment, the .control device includesa ferromagnetic core having three legs, the third of which is saturableaccording to the current in a control winding on that leg. The otherlegs are wound with the primary and secondary windings, the latter beingwound partially on each of the primary and secondary legs to counter anyemf induced by current variations in the control winding.

The foregoing control device may be considered a basic unit, or module,which represents a single phase, alternating current, welding machine.However, single phase machines of greater capacity can be obtained byconnecting in parallel two or more basic control units, and rectifiercircuits at the output can be used to convert the output to directcurrent. Basic modules can be connected to have their primary magneticcircuits in various multiphase arrangements and both primary andsecondary circuits can be connected, for example, in polyphase delta orstar forobtaining welding machines with increased capacity and fed withthree-phase current.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of theinvention, together with the objects and advantages thereof, referencemay be made to the following detailed description, and to the drawingsin which:

according to the present invention;

FIG. 2 is an electrical schematic diagram of an electronic controlcircuit suitable for use with the invention;

FIG. 3 is an exploded perspective view of the main components of asingle phase, alternating current device according to the invention andhaving one module of the type represented in FIG. 1;

FIG. 4 is an electrical diagram of a single phase, alternating currentdevice according to the invention;

FIG. 5 is an exploded perspective view of the main components of asingle-phase, alternating current device implementing two modules of thetype depicted in FIG. 1;

FIG. 6 is an electrical system diagram of the welding machineillustrated in FIG. 5;

FIG. 7 is an exploded perspective view of the components of asingle-phase, direct current-alternating current welding machine, usingtwo modules of the type depicted in FIG. 1;

FIG. 8 is an electric system diagram corresponding to the weldingmachine shown in FIG. 7;

FIG. 9 is an exploded perspective view of the main components of a threephase, direct current, welding machine with two modules;

FIG. 10 is an electrical system diagram of the welding machine shown inFIG. 9 connected on T" or Scott in the primary and intwo-phase star atthe secondary;

FIG. 11 is an exploded perspective view of the main components of athree phase, direct current, welding machine with three modules; and IFIG. 12 is an electrical system diagram corresponding to the weldingmachine shown in FIG. 11 and connected in delta at the primary andhexaphase star at the secondary.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION The principleof operation of the invention is best described by reference to thebasic element, or module, of the invention as shown in FIG. 1. Thecontrol module includes a magnetic nucleus, or core, having three legsP1, P2 and P3, respectively, closed through upper and lower shoulders Iand 2 which connect the legs. A primary winding 3 is wound on the legPlwhich, in turn, is connected to be fed from input lines L1 and L2. Onthe same leg P1 is wound part of the secondary winding 4 which, togetherwith the primary winding 3 and core, forms a transformer. Terminals 5and 6 correspond to the portion of the secondary winding wound on thecommon leg P1 with the primary winding 3.

The other part-of the secondary winding 4 is wound on the leg P2 and isconnected in series with the first part on the leg PI. Its terminals arenumbered 6 and 7, respectively, so that the series output of thesecondary winding 4 appears across the secondary terminals 5 and 7.

In accordance with the invention, an auxiliary, or control, winding 8 iswound on the third leg P3 of the core, having its terminals 9, 10connected to an electrical control circuit 11 via circuit terminals 12and 13, respectively, as can be seen in- FIG. 1. This electronic circuit11 comprises a controllable electronic switch, such as a thyristor, andits associated control circuitry. Upon conduction of the thyristor byactivation of the gate electrode coupled to the terminal 16, current ispermitted to flow in the current loop that includes the control winding8 and the path between the cathode 15 and the anode 12.

Control by the electronic circuit 11 of the current circulating throughthe control winding 8 is obtained by adjustment of the potentiometer, orvariable resistance, P that is intercon nected with the electroniccircuit 11 through the terminals 17 and 18. As will appear shortly, thecircuit 11 adjusts the duty cycle of current through the controlwinding.

The operation of the basic module of FIG. 1 is as follows.

winding 3, connected to a power source through L1 and L2,

the flux flows in a primary magnetic circuit including the legs P1 andP2 and the shoulders 1, 2. Another magnetic loop exists between the legsP1 and P3 and the shoulders I and 2. As a result of flux generated bycurrent through the primary winding, an electromotive force is inducedin the secondary winding 4 and appears between the terminals 5 and 7,the value of this emf being proportional to the number of relative turnsof the transformer windings and to the amount of flux circulatingthrough legs P1 and P2, respectively. A small amount of flux circulatesin the magnetic circuit of P1 and P3 and induces an electromotive forcebetween the terminals 9 and 10 of the control winding 8.

Under load, the normal tendency of the flux emanating from the primarywinding 3 is to by-pass the secondary leg P2 and circulate through theleg P3 containing the auxiliary winding 8. When the load is applied tothe secondary winding 4, between terminals 5 and 7, there is anopposition to the flow of the flux through the intermediate leg P2 andthe flux therefore tends to travel to the axuiliary leg P3, increasingthe electromotive force induced between the terminals 9 and 10 of thecontrol winding. Consequently, the invention takes advantage of thisnatural tendency to control the welding current, providing a simplesystem that is both economic and of .great versatility for using theauxiliary winding 8 as a gate. The current which will pass through thesecondary winding 4 will be directly related to the current which passesthrough the auxiliary winding 8.

The foregoing is achieved through operation of the thyristor 14, whichregulates the current that flows through the control winding. Thiscurrent is effective to autosaturate the leg P3 in greater or lesserdegree, depending on the period and duration of conduction of thethyristor 14. The degree of saturation of the leg P3 determines the emfinduced and, therefore, indirectly regulates the amount of flux throughthe intermediate leg P2 when the secondary winding 4 is under load.

To obtain a given current in the secondary of the transformer, thepotentiometer P of the electronic circuit 11 need only be adjusted toallow a greater or lesser conduction through the device 14. As noted,the conduction through the winding 8 establishes the degree ofautosaturation of the leg P3 and thereby the degree to which flux isforced to flow through the leg P2, proportionately. Since the thyristor14 conducts unidirectionally, in accordance with its conduction dutycycle, there is a half wave component of the magnetic flux through eachof the legs P1 and P2. Since, however, the secondary is wound partly onlegs P1 and P2, the induced emf at the output of the secondary winding,owing to the half wave current drawn from the primary and shared by legsP1 and P2, is practically nullified by properly selecting the directionof winding, the relative turns and the cross-sectional areas of the corelegs.

The description and operation of the electronic device which regulatesthe timing and duty cycle of conduction of the thyristor 14 in FIG. 1 isillustrated in schematic detail in FIG. 2. Once the desired amperage hasbeen preselected through setting of the potentiometer P (FIG. 1), theprimary winding being connected to L1 and L2 and the welding currentload being present on the secondary between terminals 5 and 7, anelectromotive force set up by the open circuit (no load) magnetizationflux through P3 is induced between the terminals 9 and 10. Since thislatter is interconnected between the terminals 12 and 13 and through theresistance 19, which produces a drop in voltage between the points 12and 17, a

I voltage regulated by the zener diode is established between 22, thetransistor 23 forms a relaxation oscillator, the capacitor 22periodically discharging through the emitter E of the transistor 23, thebase B1 and the resistance 25. As a result, a series of synchronizedpulses appears between the points 16 and 13, which in turn is impressedbetween the gate and cathode of the thyristor 14. The timing and periodof conduction of the thyristor 14 is thus a function of the voltagepulses developed across the resistance 25, and these pulses are thuseffective to regulate the average autosaturation current of the leg P3and, consequently, the current through the secondary winding 4 (FIG. 1).

In FIG. 2, the diode 26, the resistance 27, the capacitor 28 I and theresistance 29 comprise a stabilization circuit to eliminate parasiteconduction of the thyristor.

DESCRIPTION AND OPERATION or A SINGLE PHASE, ALTERNATING CURRENT WELDINGMACHINE WITH A SINGLE MODULE As observed from the illustration of FIG.3, the complete welding machine according to the invention is formed ofthe following physical parts (assigned different identifying numeralsthan the corresponding electrical components): a main base 30, a weldingtransformer or module 31, (as described in FIG. 1), an electronic device32 (as represented by the typical circuit of FIG. 2) for the thyristor,a controlled silicon rectifier or other thyristor 33, a heat sink 34 forthe thyristor, a main switch 35, a back cover 36, a cover 37, a frontcover 28, an adjusting potentiometer or welding amperage selector 39 andthe output terminals 40.

Referring to the diagram of FIG. 4, the operation of the machine is asfollows: When the primary of the transformer module, whose terminals are41 and 42 in FIG. 4, is connected to the lines L1 and L2 through theswitch 43, an electromotive force is induced between the terminals 5 and7 of the secondary winding 4 and also at the ends 9 and 10 of thecontrol winding 8.

As the load appears on the secondary due, for example, to a welding arc,the magnitude of current flowing through the secondary 4 depends on thedegree of autosaturation of the 13 is obtained, charging with more orless speed the capacitor g 22 in FIG. 2.

leg P3 of the transformer 31. This condition is obtained by theadjustment of the potentiometer P connected to the electronic circuit11, which sends signals representative of the selected condition to thegate 16 of the thyristor 14. Thus, the operation of the welding machinecorresponds to that described hereinabove in regard to the maintransformer, as well as to that explained in connection with theelectronic control circuit l1 ofFIG. 2.

DESCRIPTION AND OPERATION OF A SINGLE PHASE, ALTERNATING CURRENT WELDINGMACHINE WITH TWO MODULES CONNECTED IN PARALLEL As previously pointedout, one of the main advantages of the present invention is that itallows the standardization of the main components in welding machines ofdifferent capacities and types of connection, both single phase andpolyphase. This allows the use of the same basic unit, or weldingtransformer, to manufacture a great variety of welding machines. Thefollowing description corresponds to a single-phase, alter natingcurrent welding machine constructed with two such transformers connectedin parallel for obtaining double current capacity with a singleregulating control system.

Referring to FIG. 5, the main components are seen to be: the main base30, two transformers or modules 31 and 31', a single electronic circuit32, a thyristor 33, a heat exchanger 34 for the thyristor, a main switch35, a back cover 36, a cover 37, a front cover 38, a potentiometer 39and the terminals 40.

The operation of the machine is substantially identical to that 11through the terminals 18 and 17, and the gate of thyristor 14 is joinedto the circuit through the terminal 16. In this manner, by adjusting thepotentiometer P, the welding current can be regulated, autosaturating ingreater or lesser degree the control winding of each module. Since theyare connected in parallel, the total current available is regulated.

FIG. 7 illustrates the main components of an improved machine similar tothat described in connection with FIGS. and 6. Thus, the main componentsare identical in all respects, except for the addition of the maindiodes 44, associated heat exchanger 45, and a reactor 46.

The operation of the machine is similar to that described above for theparallel connection, and differs only in the connections of thecomplementary parts.

Turning to FIG. 8, the primaries 3, 3' of both modules are connected tothe lines L1 and L2 through the bi-polar switch 43 and each of thesecondary windings 4 and 4' has a center tap connected to the outputterminal 7 through the filtering reactor 46. The ends of the windings 4and 4, joined in parallel, are connected-to the output terminal 5through the main output diodes 44. The control windings 8, 8 of bothmodules are regulated through the single thyristor 14.

WELDING MACHINE HAVING THREE PHASE SUPPLY AND DIRECT CURRENT OUTPUT, T"OR SCOTT PRIMARY AND BIPHASE STAR SECONDARY The versatility of thesingle electronic control and universal module connected in differentforms, as has been previously mentioned, allows the economicconstruction of single-phase and polyphase machines with differentcapacities, depending on the number of modules that they contain. Themain components of a three-phase machine constructed with two modulesconnected in Scott and with a direct current output are pictoriallyillustrated in FIG. 9. In addition to the components described inconnection with FIGS. 7 and 8, this machine uses a dual circuit controldevice 32 and an additional thyristor 33', and heat exchanger 34,together with added main diodes 44' and the heat exchanger 45. All otherelements are essentially the same.

Operation of the three-phase machine is basically the same as for asingle-phase, single-module machine, except that the control windings ofeach module operate at 90 (electrical) phase difference and areregulated through two separate thyristors and two electronic circuits ofthe device 32. Preferably, the adjustable potentiometer P includesseparate but mechanically ganged resistances, one for each circuit.

The multiphase connections for the FIG. 9 machine are best appreciatedfrom FIG. 10. A primary winding of the first transformer or module,having terminals 47 and 48, is connected in T" or Scott with the primaryof the second module. The second primarys terminals are 49 and 50,center-tapped at 48. The terminals 47, 49, and 50 are connected to L1,L2 and L through a three-phase switch 43.

The secondary winding of the first module, whose ends are 51 and 52, iscoupled through two diodes 53 and 54 to a common output terminal 65. Itscenter tap 55 is connected to one end of an inter-phase transformer 56.The secondary winding of the second module, having terminal ends 57 and58, is similarly coupled through diodes 59 and 60 to the output terminal65, its center tap 61 likewise being connected to the other end of theinter-phase transformer 56. The center tap 62 of the inter-phasetransformer is connected to the second output terminal 63 through thefiltering reactor 64, corresponding to the inductance of the reactor 46.

The control windings 66 and 67 of both modules are connected to a dualelectronic control device 68, each part of which is connected in amanner already explained with respect to the single-phase machines. Ofcourse, each circuit is regulated by adjustment of a respectivepotentiometer, P1 and P2.

THREE MODULE WELDING MACHINE, THREE-PHASE DELTA PRIMARY, DOUBLEHEXAPHASE STAR OUTPUT FIG. 11 is representative of one of the morecomplex polyphase connections of the control modules. In this case, theprimaries are connected in delta and the secondaries form a double starhexaphase arrangement. The main components of this machine are: the mainbase 30, the modules 31, 31' and 31", the triple electronic device 32",three thyristors 33, 33 and 33", and associated heat exchangers 34, 34and 34", the main diodes 44, the heat exchanger 45, a back cover 36, a

front panel 38, the output terminals 40, an inter-phase 'transformer 70and the filtering reactor 46.

Again, the operation of this machine is similar to that with a singlemodule, except that it comprises a triple electronic control device forthe respective three separate thyristors. The adjustment of the machineis carried out through three-phase potentiometer providing mechanicallycoupled wiper arms for the three resistances.

The connections of this version are illustrated in FIG. 12 and are asfollows: The primaries of the three modules 3', 3' and 3", respectivelyare connected in delta for connection of the three junctions 71, 72, and73 the line terminals to L1, L2 and L3, respectively, through athree-phase switch 43". Since each of the secondaries of the threemodules is divided into two symmetrical sections, the first sections ofthe three modules, having the respective end terminals 74, 75 and 76,are connected to the anodes of the diodes 77, 78'and 79, respectively,while the other ends 80, 81 and 82 of the secondary windings are joinedto form the neutral terminal 83 of a first star circuit.

In the same manner, the second parts of the secondary windings arearranged so that their respective end terminals 84, 85 and 86 areconnected to the anodes of diodes 87, 88 and 89, respectively, whereasthe other ends 90, 91 and 92 are joined at a common terminal 93 to forma second star, electrically 180 from the other star. The cathodes of thesix diodes are joined to form the positive output terminal 94. Theneutral terminals 83 and 93 are connected to the end terminals of theinter-phase transformer 95, whose center tap 95' is connected to thenegative output terminal 96 through a filtering inductance 97 of thereactor unit 46.

The control windings in the three modules 98, 99 and 100 are connectedto a respective circuit of the electronic device 101 and each circuitmay be identical to the device shown in FIG. 2. Each winding havingrespective terminal pairs 102, 103; 104, 105; and 106, 107 is coupled tothe proper terminals of a respective control circuit. (The terminals ofthe control circuits bear numerals corresponding to those in FIGS. 1 and2 and the connections are the same as described there). As before, thepotentiometer 108 contains three electrically separate but mechanicallyjoined sections whose terminals 17 and 18 are connected to the firstsection, 17 and 18 to the second section 17" and 18 to the third sectionof the electronic control device 101.

In operation, the machine of FIG. 11 is substantially identical to thatof a single module machine, except that it operates at relative phasebetween each module, providing a three-phase, full wave rectified(hexaphase) output.

Although the invention has been described with reference to preferredembodiments, many modifications and variations of such embodiments maybe made within the skill of the art. For example, electronic devicesother than thyristors may be implemented in the control winding circuit,and other means of controlling the average control winding current areavailable. Thus, instead of varying the current duty cycle, the currentamplitude may be regulated. All such modifications and variations are,accordingly, intended to be included within the scope of the appendedclaims.

What is claimed is:

1. An improved electrical welding machine, comprising:

a magnetic nucleus with three legs;

a primary winding wound on the first leg;

a secondary winding wound partly on the first leg and partly on thesecond leg;

an auxiliary winding wound on the third leg, the latter being saturable;and

an electronic control circuit connected to the auxiliary winding forregualating the amount of flux in the auxiliary winding and the fluxthrough the secondary winding,'said electronic circuit including a firstcircuit including voltage reference means and unidirectional conductingmeans for establishing a bias voltage at a terminal;

a second circuit means including semiconductor switch means having acontrol electrode connected to charge storage means;

controllable resistence means connected intermediate the terminal andthe charge storage means to provide a varying potential at the controlelectrode, said switch means operable to discharge periodically thestorage means in response to such varying potential; and

means responsive to the periodic discharge of the storage means forregulating theconductionvof the electronic switch means.

2. An improved electric welding machine according to claim 1, furthercomprising:

a second primary winding connected in parallel with the first primarywinding;

a second secondary winding connected in parallel with the firstsecondary winding; and

a second auxiliary winding connected in parallel with the firstauxiliary winding for control by said circuit.

3. An improved electric welding machine according to claim 2 furthercomprising and connected to provide a multiphase output signal,

each secondary winding providing dual end terminals, and including twosections of which a corresponding section of each winding is connectedin a first star with each section providing one of said end terminals,and the other corresponding sections are connected in a second star,with each other section providing another of said end terminals;

unidirectional conducting means connecting said end terminals to a firstoutput terminal;

means interconnecting the two secondary windings at an intermediatepoint on each thereof and providing a second output terminal;

three electronic control means;

a control winding in magnetic circuit with each primary winding, eachcontrol winding being responsive to one of the electronic control meansto control the magnetic coupling between the associated primary andsecondary windings; and

means cooperative with the electronic control means to vary the magneticcoupling between the primary and secondary windings.

1. An improved electrical welding machine, comprising: a magneticnucleus with three legs; a primary winding wound on the first leg; asecondary winding wound partly on the first leg and partly on the secondleg; an auxiliary winding wound on the third leg, the latter beingsaturable; and an electronic control circuit connected to the auxiliarywinding for regualating the amount of flux in the auxiliary winding andthe flux through the secondary winding, said electronic circuitincluding a first circuit including voltage reference means andunidirectional conducting means for establishing a bias voltage at aterminal; a second circuit means including semiconductor switch meanshaving a control electrode connected to charge storage means;controllable resistence means connected intermediate the terminal andthe charge storage means to provide a varying potential at the controlelectrode, said switch means operable to discharge periodically thestorage means in response to such varying potential; and meansresponsive to the periodic discharge of the storage means for regulatingthe conduction of the electronic switch means.
 2. An improved electricwelding machine according to claim 1, further comprising: a secondprimary winding connected in parallel with the first primary winding; asecond secondary winding connected in parallel with the first secondarywinding; and a second auxiliary winding connected in parallel with thefirst auxiliary winding for control by said circuit.
 3. An improvedelectric welding machine according to claim 2 further comprisingunidirectional conducting means joining the terminal ends of thesecondary windings to a common point forming a first output terminal;means electrically interconnecting the secondary windings at anintermediate point on ends thereof; and a filtering reactor connected atone side to the interconnecting means and providing at the other sidethereof a second output terminal.
 4. An improved electric weldingmachine, comprising: three primary windings connected in delta andinterconnected for excitation from a multiphase source; a secondarywinding associated with each primary winding and connected to provide amultiphase output signal, each secondary windinG providing dual endterminals, and including two sections of which a corresponding sectionof each winding is connected in a first star with each section providingone of said end terminals, and the other corresponding sections areconnected in a second star, with each other section providing another ofsaid end terminals; unidirectional conducting means connecting said endterminals to a first output terminal; means interconnecting the twosecondary windings at an intermediate point on each thereof andproviding a second output terminal; three electronic control means; acontrol winding in magnetic circuit with each primary winding, eachcontrol winding being responsive to one of the electronic control meansto control the magnetic coupling between the associated primary andsecondary windings; and means cooperative with the electronic controlmeans to vary the magnetic coupling between the primary and secondarywindings.